This invention relates to methods of and systems for controlling a hydraulic turbine, such as a turbine or a reversible pump-turbine during operation thereof, and more particularly it is concerned with a control method and system of the type described which has particular utility for controlling the operation of a hydraulic turbine spaced apart a long distance from upper and lower reservoirs or having an elongated headrace and a tailrace.
A hydraulic generating station of the prior art comprises an upper reservoir, a penstock connected to the upper reservoir through an inlet gate, a turbine connected to the penstock through an inlet valve, a draft tube connected to the turbine, a draft gate at an outlet of the draft tube, a lower reservoir and a generator driven by the turbine. The turbine comprises a runner and a plurality of movable guide vanes around the runner, the guide vanes serving as flowrate control means for controlling the flowrate of water flowing through the turbine. In normal turbine operation, the inlet gate, inlet valve and draft gate are set at respective predetermined openings, and the flowrate of water introduced into the turbine is controlled only by the guide vanes. More specifically, changes in the speed of the generator resulting from changes in the load of the generator are sensed to open and close the guide vanes in accordance with variations in the load, to thereby control the flowrate of water introduced into the turbine.
In a hydraulic generating station of this construction, a sudden drop in the load of the generator causes the guide vanes to be immediately closed to reduce the flowrate of the water inflow in accordance with the variation in the load. In such case, a sudden drop in the flowrate of the water inflow causes a sudden rise in pressure on the upstream side of the turbine and a sudden drop in pressure on the downstream side thereof. The rise in pressure (.DELTA.P.sub.U) and the drop in pressure (.DELTA.P.sub.L) generally have the following relation: ##EQU1## where k: constant.
L.sub.U : length of penstock. PA1 .DELTA.V.sub.U :change in flow velocity in penstock. PA1 L.sub.D : length of draft tube. PA1 .DELTA.V.sub.L : change in flow velocity in draft tube. PA1 .DELTA.t: a short time interval. PA1 1. To close (or open) guide vanes at a sufficiently low velocity. PA1 2. To provide the turbine with a bypass water passage mounting a flowrate control valve operative to open and close as opposed to the guide vanes; PA1 3. To increase the diameters of the penstock and the draft tube to reduce velocity of the water flow; and PA1 4. To provide the water channel system with a surge tank.
As is clearly seen in equations (1) and (2), the larger the lengths of the penstock and draft tube, the higher are the rise and the drop in pressure. It is essential that the penstock be manufactured with a thickness large enough to have sufficient strength to withstand the sudden rise in pressure. The sudden drop in pressure in the draft tube has the possibility of giving rise to the phenomenon of water column separation which is not desirable. Thus closing of the guide vanes too fast must be carefully avoided.
To reduce the variations that might occur in hydraulic pressure, various proposals have hitherto been made. They include:
None of these proposals of the prior art for reducing the variations that might occur in hydraulic pressure have necessarily been successful, more specifically, even if they are satisfactory in either respect, performance or cost, they are not necessarily successful in the other respect and they leave much to be desired for the simultaneous satisfaction.